The formation of semiconductor devices such as dynamic random access memories (DRAMs), static random access memories (SRAMs), microprocessors, and logic devices requires the manufacture of a plurality of word lines and/or transistor gate stacks over the surface of a semiconductor wafer. In the recent past, the word line was formed using polysilicon as the sole conductor for the word line. As line widths continued to decrease, however, the conductivity of the polysilicon was not sufficient and the resistance of the word line became too great to produce a reliable device with desirable electrical properties. To overcome this problem with polysilicon, a tungsten silicide (WSix) layer was formed over the polysilicon to decrease the resistance of the word line and to increase conductivity. However, as line widths have continued to decrease, the conductivity of the polysilicon and tungsten silicide layers became insufficient for the word line.
A current design of a transistor gate stack is illustrated in FIG. 1, which depicts the following structures: a semiconductor wafer 10 having doped regions 12 therein; shallow trench isolation (STI) 14; gate oxide 15; a control gate comprising conductively-doped polysilicon 16, tungsten nitride (WNx) 18, and tungsten (W) 20; silicon nitride (Si3N4) capping layer 22; first silicon nitride spacers 24; and second silicon nitride spacers 26. Various other structures may also be present in the device represented by FIG. 1 which are not immediately germane to the present invention and, for simplicity of explanation, are not depicted.
During functioning of the transistor stack depicted in FIG. 1, the polysilicon 16, tungsten nitride 18, and tungsten 20 layers together function as the transistor control gate and word line for the semiconductor device. The tungsten metal layer provides greatly improved conductivity over previous devices which used polysilicon alone or polysilicon and tungsten silicide to provide improved conductivity of the word line. The conductive tungsten nitride layer, while less conductive than the tungsten, prevents the polysilicon from reacting with the tungsten layer which would form tungsten silicide WSix. This WSix layer is avoided because it forms with an irregular thickness, is difficult to remove during formation of the transistor gate stack, and has a higher resistance than the tungsten nitride. If the tungsten nitride layer is not provided and the WSix layer forms between the polysilicon and tungsten, it requires an over etch to ensure that the thicker portions of the WSix are removed. This may require etching into the polysilicon underlying the thinner portions of the WSix layer before the thicker WSix portions are completely removed and results in an over etched polysilicon layer. Over etching the polysilicon at this step forms pits in the polysilicon. Then, when the polysilicon is etched after forming first nitride spacers 24, these pits are carried through the polysilicon into the gate oxide then into the substrate 10. It is well known that pitting the substrate is to be avoided as it negatively affects the electrical characteristics of the substrate and devices formed thereon.
A problem which may occur with the FIG. 1 structure is that the tungsten nitride 18 can decompose, and free nitrogen may react with the polysilicon 16 to form a thin insulative silicon nitride dielectric layer. This dielectric layer reduces the conductivity between the polysilicon 16 and the tungsten nitride 18, and thus reduces the conductivity between the polysilicon 16 and the tungsten 20. Such an effect will increase the vertical contact resistance of the via, and may degrade the high frequency response of the device. This may result in a device which uses excessive power, has a reduced speed, and possibly a partially functional and unreliable device or a completely nonfunctional device.
A method for forming a semiconductor device, and a semiconductor device having a particular structure, which reduces or eliminates the problems described above would be desirable.